Author:

V.A. Izzo(University of California)

Extended-MHD simulations of disruption mitigation in DIII-D demonstrate that
both pre-existing islands (locked-modes) and plasma rotation can
significantly influence toroidal spreading of impurities following massive
gas injection (MGI). Given the importance of successful disruption
mitigation in ITER and the large disparity in device parameters, empirical
demonstrations of disruption mitigation strategies in present tokamaks are
insufficient to inspire unreserved confidence for ITER. Here, MHD
simulations elucidate how impurities injected as a localized jet spread
toroidally and poloidally. Simulations with large pre-existing islands at
the q$=$2 surface reveal that the magnetic topology strongly influences the
rate of impurity spreading parallel to the field lines. Parallel spreading
is largely driven by rapid parallel heat conduction, and is much faster at
low order rational surfaces, where a short parallel connection length leads
to faster thermal equilibration. Consequently, the presence of large
islands, which alter the connection length, can slow impurity transport; but
the simulations also show that the appearance of a 4/2 harmonic of the 2/1
mode, which breaks up the large islands, can increase the rate of spreading.
This effect is seen both for simulations with spontaneously growing and
directly imposed 4/2 modes. Given the prevalence of locked-modes as a cause
of disruptions, understanding the effect of large islands is of particular
importance. Simulations with and without islands also show that rotation can
alter impurity spreading, even reversing the predominant direction of
spreading, which is toward the high-field-side in the absence of rotation.
Given expected differences in rotation for ITER vs. DIII-D, rotation effects
are another important consideration when extrapolating experimental results.

*Work supported by US DOE under DE-FG02-95ER54309.

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2016.DPP.YI2.2